1. Field of the Invention
The present invention relates to an image the reading apparatus which reads an image of an original being conveyed on a convey pass, such as a copier, facsimile, or document scanner.
2. Description of the Related Art
Conventionally, an image reading apparatus includes an original feed tray on which a plurality of originals are placed, a pickup roller for taking the originals on the original feed tray into a convey pass, a feed roller that follows the pickup roller to feed the originals to the convey pass, and a retard roller paired with the feed roller to separate the fed originals one by one.
Each original fed to the convey pass is conveyed downstream by a registration roller pair and a convey roller pair, and its image is read by an image sensor while being conveyed around the image sensor. Thereafter, the original is further conveyed downstream by a convey roller pair, discharged by a discharge roller, and stacked on a stacking tray.
The image sensor includes a light source that illuminates the original, and light-receiving elements that convert reflected light from the original into electric signals. The image sensor reads the image of the original by scanning the original in synchronization with the movement of the original being conveyed on a contact glass (a guide member) of the image sensor.
At this point, foreign objects carried into the convey pass along with the original, such as paper dust originated out of the original itself and roller crumbs scraped off from the rollers conveying the original, may attach to the contact glass.
When reading a original, the image sensor is fixed at a predetermined position. Therefore, a foreign object such as dust remaining on the light path of the incident light of the image sensor will be read together with the original. The foreign object blocks the incident light of the image sensor, and a black or colored streak occurs in the read image and degrades the read image quality.
Therefore, on the occurrence of a streak in the read image, a user needs to clean the contact glass provided in the image reading apparatus and have the original read again.
To solve this problem, there has been proposed a technique for preventing foreign objects such as paper dust from frequently attaching to the contact glass by using an airflow generator provided upstream from the image sensor to send airflow to interspace between a original being conveyed and the contact glass (see Japanese Laid-Open Patent Publication (Kokai) No. 2000-151913).
In this proposal, a sensor for sensing a original is disposed at each of a paper feed position upstream from the contact glass and a paper discharge position downstream from the contact glass. Control is performed in such a manner that the driving of the airflow generator is started when the leading edge of a original is sensed at the paper discharge position, and the driving is stopped when the trailing edge of the original is sensed at the paper feed position. In this manner, flutters of the leading and trailing edges of the original caused by the airflow are prevented so as to avoid degradation in the read image.
However, with the technique of the above Japanese Laid-Open Patent Publication (Kokai) No. 2000-151913, a rigid original or a original with wrinkles, creases, etc., being conveyed tends to contact a portion of the contact glass opposed to the image sensor. This contact causes contaminants to attach to the contact glass.
Once contaminants attaches to the contact glass, the adherence increases because of the adhesion of the contaminants itself, static electricity, and so on, and airflow stronger than airflow capable of blowing off the floating contaminants is needed. This requires increasing the wind volume and wind pressure produced by the airflow generator, which will be a cause of preventing downsizing of the apparatus.
In addition, the driving of the airflow generator can not start until the leading edge of the original reaches the paper discharge position downstream from the contact glass. Therefore, during the period before the start of the driving of the airflow generator paper dust produced from the original or contaminants attached to the original cannot be prevented from attaching to the contact glass.